Acute leukemias that bear chromosomal translocations at 11q23 possess rearrangements of the Mixed Lineage Leukemia gene (MLL, HRX, ALL-1). More than 40 different MLL translocations have been reported, but the t(4;11) (MLL-AF4) is particularly common in leukemias diagnosed as ALL or mixed-lineage leukemia. Patients with MLL-AF4 leukemias have a poor prognosis. This is particularly true for infant leukemia where approximately 80% of cases will harbor rearrangement of the MLL gene. We have recently developed a conditional mouse model of Mll-AF4 ALL that recapitulates the gene expression profiles and histone methylation profiles of human MLL-AF4 ALL. Experiments described in this proposal will build upon these previous studies and characterize leukemia development with a particular focus on histone methylation. We will determine which cell types are permissive for Mll-AF4 leukemia development including hematopoietic stem cells (HSC) and early lymphoid committed cells. We will also determine if MLL-AF4 leukemia cell survival is dependent upon the histone methyltransferase Dot1L. These studies will provide a highly detailed characterization of the cells of origin of Mll-AF4 ALL, and begin to determine if histone methyltransferases are potential therapeutic targets in this disease.